Bilateral Inferior Alveolar Canal Variations Detected by Cone Beam Computed Tomography-Case Report

Abstract

Inferior alveolar canal (IAC) is an important anatomical landmark in dentistry. It is a bony canal in the mandible, one on each side and transmit inferior alveolar neurovascular bundle. However, it has been reported to have anatomical variations in its course. IAC is visible on radiographs and cone beam computed tomography (CBCT) has been identified as the one of the best methods to assess IAC preoperatively. An interesting case of IAC anatomy in a CBCT is presented here. There were accessory mandibular foramina, retromolar foramina on both sides, accessory mental foramen on the right side and lingual foramina in the midline as well as on the body of the mandible. The IAC was showing several branching within ramus as well as in the body of the mandible. This case report highlights the possible anatomical variations and importance of careful assessment of IAC in dental and maxillofacial procedures which can cause damage to the neurovascular bundle.

Introduction

Inferior alveolar canal (IAC) is an important anatomical landmark in dentistry where proper identification is of fundamental significance in diagnosis and preoperative planning. IAC is a bony canal in the mandible transmitting Inferior alveolar nerve (IAN), which is a branch of the trigeminal nerve, inferior alveolar artery, which is a branch of maxillary artery and inferior alveolar vein, which drains to Pterygoid plexus. The contents of the IAC are responsible for sensory innervation and blood supply of the mandibular teeth and periodontium [1].

The IAC begins at the mandibular foramen on the medial side of the ramus and runs obliquely downward and forward through the ramus to enter the body of the mandible and runs horizontally forward to reach the mental foramen [2]. Usually, it is bilaterally symmetrical and majority has only one major canal on each side of the mandible [3]. Although the neurovascular bundle is clearly identifiable in the cadaver studies, the IAC is visible on imaging modalities as a radiolucent zone lined by radiopaque borders [4].

However, the arrangement of IAC within the mandible have shown considerable variations among different populations, age groups and gender groups. Presences of bifid/trifid canals, accessory foramina are reported [5].

The importance of knowing the patterns of the course of the IAC is mainly to prevent injury to the vital structures during dental procedures. Iatrogenic injury to IA nerve (IAN) is a common and complex clinical problem. As the nerve lies in a bony canal, it is at risk of getting ischemic trauma and also more prone to permanent damage. The common causes of IAN injury include local anesthetic injections, third molar removal surgery, chemical and mechanical injury during endodontic, implant placement, orthognathic surgery, ablative surgery and trauma [6].

Studies have been conducted to assess the course of the IAC using cadavers, dry skulls, intraoral periapical (IOPA) radiographs and orthopantomography (OPG), computed tomography (CT), magnetic resonance imaging (MRI) and cone beam CT (CBCT) [4, 7–9]. Skull-based studies lack demographic data or have to use clinically and anatomically irrelevant landmarks [10]. OPG is the most commonly used modality but it is not always helpful in assessing the course of IAC three dimensionally; especially the buccolingual relationship [5]. CBCT is identified as superior than other conventional methods in identifying IAC [11].

In this article we present a case with several rare variations in the IAC seen on a CBCT taken for routine dental procedures.

Case Report

During a CBCT analysis for another study we came across an interesting presentation of a CBCT of a mandible, of an 18-year-old female taken at the Faculty of Dental Sciences, University of Peradeniya, Sri Lanka. The CBCT was taken for the purpose of assessing impacted 38 and 48 as of the OPG taken was showing risk features to the IAC and the 2D radiographs alone was not sufficient in preoperative planning.

CBCT Findings

It includes the whole mandible, both right and left sides, and the radiograph was of good quality and no tooth malformation or bony defects, and no other pathologies were evident. All teeth were present fully erupted except 38 and 48 being impacted.

Mandibular Foramen and Accessory Mandibular Foramina

Right-side mandibular ramus had two accessory mandibular foramina (aMF) in addition to the main mandibular foramen (MF) which was of 3.6 mm anteroposterior and 2.0 mediolateral diameter. One aMF was posterosuperior to the main foramen and 2.1 mm in diameter. The other aMF was positioned superior to MF and was 0.9 mm in diameter. Both accessory foramina were continuous with separate canals (Fig. 1a).

Fig. 1.

Mandibular foramina and mental foramina on 3D view (a–e) and coronal sections at second premolar level (f, g). a Superolateral view of right-side ramus showing MF (arrowhead), aMF (thick arrow), and RmF (thin arrow). b Superolateral view of left-side ramus showing MF (arrowhead), two aMFs (thick arrows), and RmF (thin arrow). c Enlarged view of the area marked on (b), showing two RmF (thin arrows). d Lateral view of left-side mandible showing mental foramen (thin arrow), and e right lateral view showing mental and accessory mental foramen (thin arrows). f, g The coronal section of the right-side mental foramina (white arrows) indicating the connections with the main canal

On left side there were two aMFs, one of 0.7 mm in diameter was posterior to the main foramen, and the other of 0.9 mm diameter was located on the lingula. Main foramen was 4 mm anteroposterior and 2.3 mm buccolingual diameters. They also had separate canals (Fig. 1b).

Retromolar Foramina

In the right-side retromolar triangle there was a retromolar foramen (RmF) of 1.0 mm diameter (Fig. 1a). On left side two RmFs were evident, of 0.7 mm in diameter. All had canals opening to the foramina (Fig. 1b, c).

On both sides, all three RmFs were connected to an aMF with a separate canal.

Mental Foramen and Accessory Mental Foramen

On left side there was only one mental foramen which was also in the second premolar region, and it was 2.2 mm in diameter (Fig. 1d). There were two mental foramina on right-side mandible superior and inferior to each other in the second premolar region. The one more posteroinferior was 1.5 mm in diameter and was opening to posterior direction and the anterosuperior foramen was 1.8 mm in diameter and projecting posterosuperior direction. Both foramina were connected with the main mandibular canal (Fig. 1f, g).

Lingual Foramina and Canals

Both right and left sides, in the canine and premolar regions, respectively, had lingual foramina one on each side and their accompanying canals.

In the midline of the mandible, two lingual foramina were detected, one superior and one inferior to the genial tubercles. They both were accompanied by separate canals (Fig. 2).

Fig. 2.

Lingual foramina and their canals. a shows the 3D view of the lingual aspect of the incisor region, indicating lingual foramina (L1–L4) and genial tubercles (Gt). b shows the mid sagittal section of the mandible indicating the lingual canals of L1 and L2 foramina. c show the incisive canal of right and left sides continuous over the midline in the coronal section. d–g shows the lingual canals of L1–L4 in horizontal plane

Branching Inferior Alveolar Canals

Left-Side Mandible

The main canal arise from the main MF, runs anteroinferior in the ramus and at the level of 37, 38 region it bifurcates into a superior and inferior branch. The superior canal extends toward 37. The inferior canal continues as the main canal toward premolar region and exits at 35 level through the mental foramen forming an anterior loop. The incisive branch continues further anterior in the mandible (Fig. 3a–e).

Fig. 3.

The canals of the left (a) and right side (b) mandible in 3D view, pseudo-OPGs (b, d, e, h, i), horizontal sections of right-side ramus (c) and coronal section over right third molar region (g). a–g White arrowhead: main canal, black arrowhead: mental foramen, thin arrows- accessory canals

Right-Side Mandible

The right-side IAC starting at the right MF runs straight down anteroinferior and divides into two parallel canals of similar diameters. The superior branch runs in contact with the roots of the impacted third molar and runs horizontally buccal to the molar roots and runs in anterior direction to reach the middle third of 47 and further toward 46 on buccal. The inferior canal continues inferior to the root apices and at 47 region the main canal again gives a second branch which reach the apical region of 46 which runs in the inter-radicular area of 46 and continues toward the premolar region. The main canal continues forward below the roots of 46. At 45 level, the main canal gives a branch which further bifurcate to exit through two separate mental foramina and the rest extends further anterior in the mandible as the incisive canal (Fig. 3f–i).

Discussion

Bifid and trifid MCs have been reported in the literature and also accessory canals with separate foramina [5, 12]. Anatomical variations were reported in a higher incidence with the use of high-quality imaging modalities like CBCT. A literature review has reported the occurrence of bifid mandibular canals using CBCT was 65% while in studies used OPG was between 0.08 and 0.95 percent [4].

Naitoh et al. [13] have classified bifid mandibular canals into four types as retromolar, dental, forward and buccolingual canals in a study done using CBCTs. Leite et al. [14] have observed 12% of ramification of MC in a study conducted using 250 CBCTs (500 sides) of Brazilians, where 23 out of 30 was unilateral and the rest being bilateral. In their study, retromolar type was the most common followed by forward canals and dental type. They have observed 0.4% buccolingual canals. In addition to the ramification, they have also observed 51.6% patients with large diameter incisive canals 3.2% of unilateral accessory mental foramen. In the present study we observed retromolar type, dental type and lingual canals but also a type which is not in line with the classification, where they passed along the mid-horizontal plane of the molar roots. Patil et al. [15] have observed 129 subjects with retromolar canals out of 171 CBCTs investigated. Retromolar canals have further classified to three types [16], and Gunasena et al., [17] have reported three Sri Lankan patients’ CBCTs, more compatible with the type A, retromolar canals. The present case is more compatible with type C canals, which is defined as a canal piercing temporal crest of the coronoid process [16]. This presentation of canals has been also named as temporal crest canals but instead of the retromolar region, the canal opens to the anterior of the temporal crest of the ramus [18]. Mizbah et al. [19] have reported a similar case with trifid and bifid mandibular canals on either side of the same mandible in a CBCT.

The inferior alveolar nerve which emerges from the mandibular division of trigeminal nerve, contains both motor and sensory fibers, and before it enters the mandible it gives off the motor fibers as the nerve to mylohyoid muscle. And the rest enter the mandibular foramen as IAN. Also IA artery, branching from maxillary artery enters the mandible through MF. There are studies done using gross anatomical and histological which have revealed that the accessory canals also contain neurovascular bundles [18, 20–22]. There are cadaveric studies which have shown different patterns of the distribution of IAN, including single un-branched nerve to proximal and distal/molar nerve plexuses. In the latter type, 41% cadavers have shown posterior molar plexus while 37% have shown posterior and anterior nerve plexuses [5, 20]. Cater and Keen [20] have also observed three main types of IAN arrangements within the mandible including plexus formation, in their mandible dissection study. The type 3, which they have reported, was similar to the pattern we observed in the present case, where IAN had given off two large alveolar branches posteriorly and the main nerve continue in a more inferior position toward the mental foramen. They have also observed smaller foramina of 0.1 mm diameter or greater on the posterior part of mandible, more commonly near the condyle, and retromandibular fossa, where there is insertion of muscle of mastication. Those foramina have also seen to transmit neurovascular bundles. Cater and Keen have also seen connections of IA neurovasculature with muscles of mastication, most commonly with pterygoid and temporalis muscles. Temporalis neurovascular bundles have also traced to the retromandibular foramina and inside where they have made connections with the main trunk of IAN. The dental branches supplying the molar teeth were entering lateral surface of the root rather the apices. Incisive branch was displayed to given off, just before the mental branch and further they were traced to foramina on the posterior aspect of the symphysis menti, either below or above the genial tubercles which is a similar presentation to the presenting case. Such lingual foramina were to have connections with the mylohyoid muscle [20]. The literature also has evidence that mylohyoid nerve containing sensory fibers which supply the chin and they send branches to mandible from lingual foramina [23].

The presence of branching canals with vital structure which runs more superior direction than the usual course of the IAC can complicate dental procedures like implant placement, impaction surgeries and even denture placement after tooth loss. In case the accessory canals may contain nerve branches it may complicate local anesthesia [6, 24, 25]

In the present case report, right-side mental foramen was seen to be having an accessory foramen in which the mental neurovascular bundle divide into two branches to exit. Few studies have reported the presence of accessory mental foramina in Sri Lankan population done using dry mandibles. Prabodha and Nanayakkara, [26] have reported two out of 24 hemi mandibles (8.33%) with accessory mental foramina, and Ilayperuma et al. [27] have reported 3.92%, including two as well as three mental foramina. Nanayakkara et al. [28] have reported an occurrence of 11.5% of accessory mental foramina in a study done using 26 dry mandibles.

Conclusion

Anatomical variations of the mandibular canal and teeth are not uncommon which can lead to undesirable treatment outcome. CBCT is a useful diagnostic tool for identifying such variations. Therefore, it is important to assess the IAC radiologically before attempting dental treatments in the mandible. Considering the cost and availability, CBCT can be recommended for further evaluation of the IAC in cases where routine radiography reveals risk factors. Studies with a larger population base for specific risk factor evaluation using 2D and 3D radiographic methods is the need of the hour for delivering evidence based state-of-the-art dental care to the clientele.

Funding

None

Declarations

Conflicts of interest

We declare that we have no conflicts of interest.

Human and animal rights statement

This article does not contain any studies with human or animal subjects performed by the any of the authors.